Add `sis_batch_id` to NOP fields for update

`sis_batch_id` is updated even when no other field is changed on a sis
batch import. Add `sis_batch_id` to the list of fields that will not
trigger a live-events stream update.

Test Plan:
  * Specs

closes RECNVS-239

Change-Id: I15f0b82dbbe1dc47885d3f2ce05932ba66f96c97
Reviewed-on: https://gerrit.instructure.com/137963
Tested-by: Jenkins
Reviewed-by: Rob Orton <rob@instructure.com>
Product-Review: Jonathan Featherstone <jfeatherstone@instructure.com>
QA-Review: Jonathan Featherstone <jfeatherstone@instructure.com>

app/observers/live_events_observer.rb

@@ -34,9 +34,10 @@ class LiveEventsObserver < ActiveRecord::Observer
           :account_notification,
           :course_section
 
+  NOP_UPDATE_FIELDS = [ "updated_at", "sis_batch_id" ].freeze
   def after_update(obj)
     changes = obj.changes
-    return nil if changes.except("updated_at").empty?
+    return nil if changes.except(*NOP_UPDATE_FIELDS).empty?
 
     obj.class.connection.after_transaction_commit do
     case obj

spec/observers/live_events_observer_spec.rb

@@ -27,6 +27,16 @@ describe LiveEventsObserver do
       expect(Canvas::LiveEvents).to receive(:user_updated).never
       @user.save!
     end
+    it "doesn't post events for NOP fields" do
+      account_model
+      course_model(name: "CS101", account: @account)
+      sis = @account.sis_batches.create
+
+      @course.name = "CS101"
+      @course.sis_batch_id = sis.id
+      expect(Canvas::LiveEvents).to receive(:course_updated).never
+      @course.save!
+    end
     it "does post event for actual change" do
       user_model(name: "Joey Joe Joe")